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Welcome to the official Square Foot Gardening Forum.
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We have used pine with pretty good results. Even if you need to replace them a few years down the road, which we haven't, they are so inexpensive it's not too bad. We are in So. Calif so not rough weather.

Some have used cedar fencing. Search the web and you will find some great sites about building cedar sag beds. Be sure to make them at least 6"s deep. This is from my own experience. I work at the Lowes in Corona if your in the area-stop by.